I havent read every response so if my tip is already posted please forgive me for repeating but here is what works for me: I use 987 on both the outside and the lining of the bag. I found that the bag is just not sturdy enough without it. To alleviate the problem of "thickness" when sewing it together I always cut my 987 a half inch shorter than I need to back each piece. That way when I center it on the back of the piece I have a quarter inch of fabric showing on each side all the way around. When sewing the bag together I then barely catch the edge of the 987 in the seams. I've made many bags using this method and they have all held together really well and I have no bulkiness in the seams. Hope that helps.